Screening machines, that is, machines having one or more screens for sizing, sifting or separating particulate material by vibratory, oscillatory, or gyratory screening movement, are in widespread commercial use. The screen or screens of such machines are mounted or housed within a movable enclosure called a "screen box." To prevent the escape of dust, the box typically has a top cover with an inlet opening. The material to be screened is usually fed from a source such as a hopper or elevator through a feed spout or inlet pipe, and falls through the inlet opening in the top cover, onto the top screen. The box and cover prevent the escape of dust and fines during operation and prevent the entrance of outside dirt which could contaminate the product being screened. One example of such a machine is the ROTEX.RTM. brand machine produced and sold by the assignee of this application.
In operation, screening movement applied to the box separates the different size or shape fractions of the particulate feed according to the size and shape of the screen openings. The various separated fractions are conducted from the screen box through one or more outlet conduits. Because the box (with the top cover on it) is moved in rapid screening motion, provision must be made for accommodating its motion relative to the stationary feed spout while at the same time enclosing the feed stream and airborne fines, and preventing the entrance of dirt. The same problem of accommodating relative motion arises when particles fall from a moving screen box into a fixed receiver pipe, and in other applications where one pipe moves or is disaligned with respect to another.